Reuters quotes Marines spokesman Lt. Ryan Finnegan as saying that the four were killed at the Zulu Impact Area inside the base.

"We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident," said base commander Brig. Gen. John W. Bullard. "Our first priority is to provide the families with the support they need during this difficult time."

Officials said they were investigating the cause of the accident but had no further details as yet and did not release the identities of the dead, pending notification of their families, the AP said.

The AP notes:

"The deaths come about eight months after a mortar explosion killed seven Marines during a live-fire training exercise in Nevada.

A military investigation determined human error was to blame for that accident. According to the probe's findings, a Marine operating a 60 mm mortar tube and ammunition did not follow correct procedures, resulting in the detonation of a high explosive round at the mortar position.

The investigation also determined that the mortar team involved in the accident had not conducted "appropriate preparatory training."